Implement a mutable container for building formats

Author: dkhalanskyjb

core/common/src/internal/format/Builder.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2019-2023 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.internal.format
+
+/**
+ * An append-only list of constituent formats. Can't be exposed directly due to the variance rules.
+ */
+internal class AppendableFormatStructure<T> {
+    private val list: MutableList<NonConcatenatedFormatStructure<T>> = mutableListOf()
+    fun build(): ConcatenatedFormatStructure<T> = ConcatenatedFormatStructure(list)
+    fun add(format: FormatStructure<T>) {
+        /** TODO: use the caches present in [CachedFormatStructure]. */
+        when (format) {
+            is NonConcatenatedFormatStructure -> list.add(format)
+            is ConcatenatedFormatStructure -> format.formats.forEach { list.add(it) }
+        }
+    }
+}